For the purpose of environmental improvement, reduction of harmful substances in exhaust combustion gas, such as CO (carbon monoxide), NOx (nitrogen oxide), and HC (hydrocarbon), from the internal-combustion engine has been desired. Meanwhile, reduction of greenhouse gas, i.e., CO2 (carbon dioxide), is desired for the purpose of global warming prevention. In order to simultaneously attain the goals of reduction of CO2 and reduction of CO, NOx, and HC, removal of such substances with the use of a catalyst has been examined.
As a method for reducing NOx, CO, and HC from the exhaust combustion gas of an automobile, a method for operating a three-way catalyst has been known. It is said that a three-way catalyst effectively functions when the internal-combustion engine is under rich fuel operation; however, such catalyst is not effective when the internal-combustion engine is under lean fuel operation. Thus, the effective use of such catalyst cannot be simultaneously pursued with the reduction of CO2 while the internal-combustion engine is under lean fuel operation.
As a method for reducing NOx from the exhaust combustion gas of a boiler, a method using an ammonia-based deNOx catalyst has been known. Since the method using an ammonia-based deNOx catalyst is effective under lean fuel operation, CO2 can be reduced while NOx is removed. This method, however, requires the use of an apparatus and a method for separately adding ammonia to exhaust gas.
Further, as a method for reducing NOx from exhaust combustion gas, a method involving the use of a non-ammonia-based deNOx catalyst has been examined (i.e., a method using an NOx selective reduction catalyst that allows selective reaction of NOx and CO contained in exhaust gas to reduce and remove NOx). An example of an NOx selective reduction catalyst is a gold-based catalyst.
JP Patent Publication (kokoku) No. 6-29137 (B) 1994 discloses a catalyst comprising gold particles supported on titanium oxide. Also disclosed is a method for preparing an Au catalyst wherein the pH level of a starting gold chloride solution is adjusted to 5 to 9 to prepare Au microparticles. It should be noted that a pH level exceeding 9 results in significantly deteriorated catalytic capacity.
JP Patent Publication (kohyo) 2001-508360 (A) discloses the use of a composite catalyst of aluminum oxide, zirconium oxide, cerium oxide, titanium oxide, silicon oxide, or a mixture thereof with Au and a transition metal as a catalyst for reducing nitrogen oxide.
JP Patent Publication (kokai) No. 6-219721 (A) 1994 discloses a catalyst for oxidizing CO comprising at least one member selected from the group consisting of Rh, Ru, Pd, Os, Ir, and Au incorporated into at least one oxide selected from the group consisting of CeO2, ZrO2, TiO2, and SnO2.
However, further improvement in the capacity of such catalysts for purification is desired.